Invisibility cloaks have taken a step closer to reality as a result of new research carried out by a group from the University of California Berkeley in the US.
For an object to become invisible light must be able to bend round it and this can be achieved by creating structural arrays with features smaller than the wavelength of light. Most everyday objects bend light by ‘positive refraction’, making them visible. However there is a class of materials called metamaterials which exhibit ‘negative refraction’ and these are able to alter the propagation of light.
The scientists led by Xiang Zhang use two different methods to overcome the obstacles to reversing the natural direction of light. In their first paper [Valentine et al., Nature (2008) doi:10.1038/nature07247], the team stacked up alternate layers of conducting Ag with non-conducting layers of MgF2. Patterns cut into the layers by focused ion beam milling create a bulk optical metamaterial with nanoscale features. Each pair of layers in the structure makes up one circuit and stacking them up creates a series of circuits that can respond to the incoming light. The combined effect of the ‘fishnet’ patterned structure and the influence of the interacting layers allows light to pass through the material without wasting large amounts of energy. A negative refractive index is observed for these materials at wavelengths of 1500 nm.
In their second approach [Yao et al., Science (2008) 321, 930], the team created a structure composed of Ag nanowires in a porous aluminium oxide material. The material is ten times thinner than paper but still ten times the size of a wavelength of light, making it a bulk material containing small nanowire arrays. The vertical wires only respond to the electrical field of the incoming light and not the magnetic field and thus are able to bend light with minimal energy loss. A negative refraction is observed for these materials at wavelengths of 660 nm. It must be said, however, that for light to be bent by negative refraction the material's transmission of electric fields and its response to magnetic fields must both be negative. Therefore, in this case, where the wires do not respond to the magnetic field, the effect is not, strictly speaking, true negative refraction.
For many applications, including cloaking, either of these two routes is viable. True negative refraction as seen in the fishnet metamaterial has several advantages though, one of which is reduction of interference in antennae.